Although there are many types of artificial respirators, the mainstream at present is an apparatus of the positive pressure type which applies positive pressure directly into the trachea. With this apparatus, although the artificial respiration can be positively effected, an incision of the trachea is needed, and the incision portion must be sterilized. A further disadvantage of the positive pressure type respirator is that the patient is unable to consume food or effectively speak. Another type of respirator is a negative pressure type apparatus commonly referred to as an "iron lung". The negative pressure type apparatus also has disadvantages in that it is bulky and is low in efficiency. As a result, the negative pressure type apparatus has been seldomly used in recent years. Another negative pressure type apparatus is one known as an outside-of-thorax type negative pressure artificial respirator. This apparatus includes a corset having a rigid shell for enclosing the thorax of the patient, and forms an air-tight chamber between the thorax and the rigid shell when the corset is attached. By bringing the sealed chamber into a negative pressure, the artificial respiration is carried out. Since this apparatus does not need an incision of the trachea, and can be easily used, the apparatus has recently been extensively used.
FIG. 7 shows a conventional outside-of-thorax type negative pressure artificial respirator including the corset 50 and a suction pump 51 which are interconnected by an inspiration tube 52, and a two-way directional control valve 53 is mounted in a conduit of the inspiration tube 52 so that the inspiration tube 52 can be selectively opened and closed relative to the atmosphere. During the inspiration period, the two-way directional control valve 53 is closed relative to the atmosphere to bring the pressure within the corset 50 to a negative pressure. During the expiration period, the valve 53 is opened relative to the atmosphere to return the pressure within the corset 50 to the atmospheric pressure. By controlling the pressure within corset 50 in this manner, artificial respiration is carried out.
However, in the conventional apparatus shown in FIG. 7 the directional control of the conduit by the two-way directional control valve 53 is instantaneously effected. Specifically, the pressure within corset 50 is abruptly changed between a negative pressure and the atmospheric pressure, as shown in FIG. 8. This results in a problem in that the patient is subjected to an impact which causes pain.
As described above, the conventional outside-of-thorax type negative pressure artificial respirator has a problem in that when the tube pipe connected to the corset is to be opened and closed relative to the atmosphere, the two-way directional control valve achieves the directional control of the conduit instantaneously, and therefore the pressure within the corset is abruptly changed to provide an impact and hence a pain to the patient.